Silicone hydrogel contact lenses can be manufactured economically in a mass production manner by a conventional cast-molding process involving disposable molds (e.g., PCT published patent application No. WO/87/04390, EP-A 0 367 513, U.S. Pat. No. 5,894,002, all of which are herein incorporated by reference in their entireties) or by an improved cast-molding process involving reusable molds and curing under a spatial limitation of actinic radiation (U.S. Pat. Nos. 5,508,317, 5,583,163, 5,789,464 and 5,849,810). A critical step in the production of lenses using molds is mold opening and lens releasing from the mold without damaging the lens. Subsequent to the completion of the contact lens molding process, the polymerized lens tends to strongly adhere to the mold. During mold opening and removing the contact lenses from the mold, cracks, flaws and/or tears may occur in the lenses or in the worst case the contact lenses even break totally. Contact lenses having such defects have to be discarded, resulting in lower production yield and higher production costs.
Various methods have been developed or proposed. One exemplary method is to hydrate the lens, namely, a lens-in-mold assembly after mold separation is placed in a hydration tank filled with water containing a surfactant (e.g., U.S. Pat. No. 5,264,161). However, the utilization of surfactants in a hydration bath does not provide a more effortless mold separation. Lens damage incurred during mold separation may not be minimized by hydrating lenses. Such method may not be easily implemented in a fully automated production line. Another exemplary method is to incorporate surfactants as internal mold releasing agents into molds as illustrated by U.S. Pat. No. 4,159,292. Incorporation of internal mold releasing agents in molds can decrease adhesion between lenses and molds. However, when a mold is used repeatedly, surfactants as internal mold releasing agent can be exhausted by exudation. A further exemplary method is to apply external mold releasing agents (e.g., surfactants) in the form of a film or coating onto to the molding surfaces of a mold (e.g., those disclosed in U.S. Pat. Nos. 4,929,707 and 5,542,978). When external mold releasing agents are used, a portion of the agents used for treating the molding surfaces of the mold can migrate to the surface and interior of the polymerized lens, which results in problems such as non-uniformity of the surface of the lens and turbidity. When a mold is used repeatedly, mold releasing treatment is required in each molding cycle. Such treatment can lower productivity in producing the lens. A still further exemplary method is to incorporate internal mold releasing agents into a lens-forming composition for making contact lenses. The internal mold releasing agent can be a surfactant (U.S. Pat. Nos. 4,534,916; 4,929,707; 4,946,923; 5,013,496; 5,021,503; 5,126,388; 5,594,088; and 5,753,730, herein incorporated by reference in their entireties) or a non-polymerizable polymer (U.S. Pat. No. 6,849,210, herein incorporated by reference in its entirety). By incorporation of an internal mold releasing agent in a lens-forming composition (or lens formulation), the adhesion between molds and lenses may be reduced, a relatively smaller force may be required to separate mold, and lenses may be removed from molds with less effort. However, for a specific lens material and/or a mold material, not all surfactants or polymers can be effective mold releasing agents for facilitating mold separation and lens removal from a mold, in particular, under conditions of high speed automated operations (i.e., the time interval between dosing a lens-forming material in a mold and curing the lens-forming material in the mold is relatively short, e.g., less than about 20 seconds). Although some mold releasing agents may be able to lower an averaged mold separation force for a given lens-forming material, individual mold separation forces may vary widely and become an uncontrollable factor that affects the product quality and yield. In addition, a non-crosslinkable polymer may be effective in reducing mold separation force but may have adverse effects on the properties of resultant lenses. For example, it may cause resultant lenses to be hazy and affect the refractive index of the resultant lenses. Furthermore, there are few known methods for selecting a non-polymerizable polymer as an effective mold releasing agent for a given lens-forming material.
Therefore, there still a need for a process for cast-molding contact lenses with an enhanced quality and enhanced yield achieved by reducing mold separation force and lens-mold adhesion.